The invention relates to a new type of clip and osteosynthesis plate having characteristics of dynamic compression and self-retention.
The term "dynamic compression" signifies the ability of these clips or plates to generate a compression force resulting between the two points at the level of which they are implanted and, more particularly, on each side of a bone fracture area.
The surgical clips used for the fixation of the bones and soft tissues must possess a number of essential characteristics. First, they must develop a compression which is constant over time. In addition, they must be anchored perfectly in such a way as to prevent unhooking after their implantation, this unhooking generally being due to movements of the articulation or simply of the bone on which they are implanted. Finally, the method of implantation or removal of the clips and plates must be simple, easy to implement, and cause a minimum level of osseous traumatism.
Various types of clips or plates aimed at achieving these aims are currently known. For example, in order to ensure their retention in the soft bone, clips have been proposed whose lateral branches are equipped with projecting parts or ridges, which are intended to prevent the withdrawal of the clip from the tissue (FR-A-2,525,102). However, although it is true that this type of clip eliminates any risk of unloosening from the bone to be fixed, it does not generate any dynamic compression and, in addition, the removal of the clip results in considerable osseous traumatism.
It has also been proposed, particularly in the document DE-A-2,703,529, to produce clips made of martensitic material (of alloy of the Ni--Ti or Ti--Nb type), and to confer upon the branches of the clips a shape memory capable of inducing a moving together of their ends above the austenitic temperature of the said martensitic material of which the clip is made. This "shape memory" phenomenon is due to the reversible thermoelastic martensitic transformation. This phenomenon is well known and consists in giving to a material a defined shape which is treated at a temperature above the austenitic temperature As of the material, then in giving it another shape, likewise defined, at a temperature below the martensitic temperature Ms of the said material, and finally in repeating this operation several times as a function of the nature of the alloy used, in order to give this material its definitive shape memory. This temperature Ms is below the temperature As.
However, although it is true that this type of clip provides a dynamic compression at the level of the end of the clips, this dynamic compression generally proves inadequate for the whole of the fracture at the level of which the clip is implanted, and is even in some cases damaging because this compression is asymmetrical; indeed, it brings together the deeper areas of the fracture zone and distances the superficial area of this same zone.